vi GENERAL CONSIDERATIONS ON THE TEXTURES. 



acid, hippuric acid, inosinic acid, sarkin (or hypoxanthin), leucin, tyrosin, 

 azotised biliary compounds. 



II. Substances destitute of Nitrogen, viz., fatty matters (except cerebric 

 acid), glycogen (or animal starch), animal glucose, sugar of milk, inosit, 

 lactic, formic, and oxalic acids, certain principles of the bile. 



Some of the substances now enumerated require no further notice in a work 

 devoted to anatomy. Of the rest, the greater number will be explained, as far as 

 may be necessary for our purpose, in treating of the particular solids or fluids in 

 which they are chiefly found ; but there are a few of more general occurrence, the 

 leading characters of which it will be advisable here to state very briefly, viz. : 



A. Albuminoid Principles, albumen, fibrin, and casein. Coagulable fibrin 

 spontaneously, albumen by heat, casein by rennet. Precipitated by mineral 

 acids, tannic acid, alcohol, corrosive sublimate, subacetate of lead, and 

 several other metallic salts. When coagulated, not soluble in water, cold or 

 hot, unless after being altered by long boiling ; insoluble in alcohol ; soluble 

 in alkalies ; soluble in very dilute and also in concentrated acids ; the solu- 

 tions precipitated by red and yellow prussiates of potash. 



B. Gelatinous Principles, gelatin and chondrin. Not dissolved by cold 

 water ; easily soluble in hot water ; the solution (at least that of gelatin) 

 congealing when cold. Precipitated by tannic acid, alcohol, ether, and 

 corrosive sublimate, and not by the prussiates of potash. Chondrin pre- 

 cipitated by acids, alum, sulphate of alumina, persulphate of iron, and 

 acetate of lead, which do not precipitate gelatin. 



c. Extractive Matters, associated with lactic acid and lactates. All soluble 

 in water, both cold and hot ; some in water only ; some in water and 

 rectified spirit ; some in water, rectified spirit, and pure alcohol. 



D. Fatty Matters. Not soluble in water, cold or hot ; soluble in ether 

 and in hot alcohol. 



It has been shown by Mr. Graham,* that chemical substances may be distinguished 

 into two classes the crystalloid and the colloid which differ in several important 

 characters. Crystalloid bodies, of which water, most salts and acids, and sugar, may 

 be taken as examples, have a disposition to assume a crystalline state ; their solutions 

 are usually sapid, diifluent, and free from viscosity ; they readily diffuse in liquids, 

 and pass through moist organic membranes or artificial septa of organic matter, such 

 as parchment-paper. Colloids, on. the other hand, are characterised by low diffusibility 

 and great indisposition to permeate organic septa, so that when they are associated 

 with crystalloids, the latter may be easily separated by diffusion through a septum 

 into another fluid ; i.e., by " dialysis." Colloids are, moreover, generally tasteless ; 

 they have little or no tendency to crystallize, and their solution, when concentrated, 

 is always, in a certain degree, viscous or gummy. Among the colloids may be 

 reckoned hydrated silicic acid, and various hydrated metallic peroxides, also albumen, 

 fibrin, gelatin, starch, gum, and vegetable and animal extractive matters. Several 

 substances may exist either in the colloid or the crystalloid condition. In point of 

 chemical activity the crystalloid appears to be the more energetic, and the col- 

 loidal the more inert form of matter ; but the colloids possess an activity of their 

 own, arising out of their physical properties, and especially their penetrability, 

 by which they become a medium for liquid diffusion, like water itself. Another 

 characteristic is their tendency to change; the solution of hydrated silicic acid, 

 for instance, cannot be preserved ; after a time it congeals. In this respect a 

 liquid colloid might be compared to liquid water at a temperature below freezing, 

 or to a supersaturated saline solution. This dominant tendency of the par- 

 ticles of a colloid to cohere, aggregate, and contract, is obvious in the gradual 



* Liquid Diffusion applied to analysis, Phil. Trans., 1861. 



